For a semiconductor laser used as a light source in an opto electronic integrated circuit (hereinafter referred to as "OEIC"), it is required that the threshold current below the, two electrodes be provided on the same surface and that surface be flat for integration with electronic devices such as FETs. A Transverse Junction Stripe (TJS) type laser is well known in the GaAs system producing an oscillation wavelength of 0.78 to 0.9 microns satisfying the above described requirements. This type of laser is described in detail, for example, in Japanese Journal of Applied Physics, vol. 18 (1979), Supplement 18-1, pp. 371-375. A description is given here briefly with reference to the drawings.
FIG. 3 shows a schematic cross-sectional view of a GaAs system TJS laser. In figure, reference numeral 301 designates a substrate formed of semi-insulating GaAs. A lower cladding layer 302 formed of n type AlGaAs is disposed on the substrate 301, an active layer 303 formed of n type GaAs is disposed on the lower cladding layer 302, and an upper cladding layer 304 formed of n type AlGaAs is disposed on the active layer 303. These three layers constitute a double heterostructure 310 grown by liquid phase epitaxy. Reference numeral 305 designates an n type GaAs contact layer formed on the double heterostructure 310. Reference numeral 106 designates a p type diffused region formed by selectively implanting or diffusing p type impurities into the n type GaAs layer 305 and the double heterostructure 310 reaching the substrate 301. A groove 107 is produced and removes a pn junction formed in the n type GaAs contact layer 305. P side electrode 108 and n side electrode 109 are disposed on the surface of the n type GaAs contact layer 305 respectively. Reference numeral 110 designates a laser active region formed in the active layer 303 by spreading out the p type impurities from the p type diffused region 106 to the n type region by approximately 2 microns in a drive-in diffusion.
The process for producing a conventional semiconductor laser device will be described.
A double heterostructure 310 is produced on the GaAs substrate 301 by successively growing n type AlGaAs lower cladding layer 302, n type GaAs active layer 303, and n type AlGaAs upper cladding layer 304, by such as liquid phase epitaxy (LPE). In addition, n type GaAs contact layer 305 is further grown by LPE. Next, to form p type diffused region 106, Zn as p type impurity is selectively diffused into the n type GaAs layer 305 and the double heterostructure 310 at a temperature of, for example, 650.degree. C., to reach the substrate. Thereafter a front of the p diffused region 106 is spread out, that is, diffused into the n type crystal by approximately 2 microns by heating at a temperature of 930.degree. C. Next, a pn junction produced in the n type GaAs layer 305 (not illustrated) is removed by etching a groove 107. Thereafter, p side electrode 108 and n side electrode 109 are formed and then a TJS laser is completed.
A description will be given hereinafter of operation.
When a voltage is applied to this laser making the side of the p type diffused region positive, an electric current flows and is concentrated in the junction produced in the n type GaAs active layer 303 having the lowest pn junction diffusion potential, so that a laser oscillation occurs in an active region 110 in the n type GaAs active layer 303. Laser light is radiated in a direction perpendicular to the paper on which FIG. 3 is shown.
P side electrode 108 and n side electrode 109 are provided on the opposite surfaces of the laser as in a usual laser and the two electrodes are separated from each other by the thickness of laser chip, that is by approximately 100 microns in the up-and-downward direction. On the other hand, since a source, gate and drain electrode of an FET are on the surface of the substrate, the integration of the an FET and the conventional semiconductor laser device requires the wiring between an optical device and FET include steps. This causes, however, a problem such as breakage of wiring at an edge of a step.
As is apparent from FIG. 3, the conventional laser has both the p and n side electrodes on the same surface and this is advantageous in integration with an FET in avoiding wiring breakage at an edge of step as discussed above. With respect to the current threshold which is important in laser characteristics, a threshold current of less than 20 mA is realized in the GaAs system laser, as disclosed in the above-mentioned literature.
However, the oscillation wavelength of the GaAs system laser is 0.78 to 0.9 microns, which does not coincide with the low loss wavelength region of quartz fibers which are used in optical communication. InGaAsP has been known as a material having oscillation wavelength in the low loss wavelength region (so-called long wavelength band) of 1.3 to 1.5 microns. Therefore, the TJS structure was attempted to be applied to the InGaAsP system in order to construct a high efficiency semiconductor laser which radiates laser light of long wavelength and has an appropriate structure for OEIC.
FIG. 4 is a cross-sectional view showing a structure of a TJS InGaAsP system laser which is disclosed in IEEE Journal of Quantum Electronics, vol. QE-15(1979), pp 710-713.
In this figure, reference numerals that are the same as in FIG. 3 designate the same or corresponding parts. Reference numeral 401 designates an n type InP substrate. An n type InP lower cladding layer 402 is disposed on the n type InP substrate 401. An n type InGaAsP active layer 403 is disposed on the n type InP lower cladding layer 402. An n type InP upper cladding layer 404 is disposed on the n type InGaAsP active layer 403. The laser shown in the article has an n side electrode provided on the rear surface of the element. However, a construction like the TJS laser of FIG. 3 is possible by providing a semi-insulating substrate and providing an n side electrode on an n type region at the surface of the element.
According to this conventional laser, a double heterostructure is realized by using a combination of InP/InGaAsP instead of combination of AlGaAs/GaAs. Because the energy band gap of InGaAsP is smaller than that of InP, current is concentrated in a pn junction formed in InGaAsP. The operation principle is the same as that of the GaAs system TJS laser. However, this TJS laser does not oscillate at room temperature and a 100 mA threshold value is obtained only at low temperature such as 100.degree. K. The reason why a high threshold current was required has never been clarified until now.
Our investigation has revealed the reason why the threshold current is high in the InGaAsP system TJS laser.
In the InGaAsP system the impurity concentration of Zn diffused p region is at most 5.times.10.sup.18 cm.sup.-3, which is by one order of magnitude lower than that of the GaAs system material, and the resistivity of p region is large. When the resistivity of p region is large, the voltage drop to the active layer from the electrode is large and a large voltage needs to be applied in order to make an electric current flow in the active region. Therefore, a voltage exceeding the diffusion potential is applied to the InP-pn junction outside of the active region. As a result, the leakage current flows into a pn junction in InP where no current is designed to flow, resulting in a large oscillation current threshold for the laser.
Due to being constructed as described above, the conventional long wavelength TJS laser has disadvantages in that it can not oscillate at room temperature and its oscillation current threshold is too high to be practical.